This article examines Reinhold Niebuhr's contributions both to twentieth-century liberalism and to reflection on liberalism's relationship to political ethics. These contributions are, I argue, twofold. First, Niebuhr's account of moral psychology chastens liberal ambitions and assumptions, offering a more “realistic” liberalism (while avoiding certain weaknesses of “realist” theories). Second, Niebuhr's thought points to the importance of ethos—the combination of dispositions, temperament, and perceptions that shape individual conduct—in defining liberalism and determining the quality of political action. In articulating an ethos of humility, moderation, skepticism, irony, and self-restraint, Niebuhr offers a reformulation and defense of liberalism, and a warning against dangers that threaten liberalism both from within and without.